Normally you would expect to have been teaching for a few years before mentoring. There is no point in offering help and guidance if you are not secure in your own ability yet. But sometimes in small schools, or those with few specialists, you may be obliged to assume this role. In this case, make sure you are offered training, and try to arrange for another member of staff to support you.
As for extra pay, much will depend upon whether you are offered time for the role or expected to add it to your existing duties. Mentoring of NQTs is not specified in the teachers' pay and conditions document, but teachers can be required to contribute to others' professional development. The supervision of students on initial teacher training courses is covered by different rules.